(1) Field of the Invention
The present invention relates to an aircraft power plant and to an aircraft having the power plant. More particularly, the invention relates to a power plant having a connection device for connecting together a main gearbox (MGB) and an engine.
In particular, the invention lies in the field of rotorcraft power plants having a piston engine, and more particularly a diesel engine.
(2) Description of Related Art
Rotorcraft are generally fitted with at least one rotor that is driven in rotation by a power plant. The power plant is provided with at least one engine and with a main gearbox, said main gearbox being interposed between the engine and the rotor.
The engine may be a turbine engine connected by a drive train to the main gearbox.
Such a drive train comprises a transmission shaft extending between an outlet shaft of the engine and an inlet shaft of the main gearbox. The transmission shaft may comprise one or more segments.
While the power plant is in operation, the engine and/or the main gearbox may be subjected to small movements.
The main gearbox may be suspended to the structure of an aircraft by a conventional suspension system. The suspension system may for example comprise suspension bars and suspension means. Reference may be made to the literature in order to obtain teaching on this topic.
Likewise, an engine may be attached to the structure of the aircraft by legs, and where appropriate by energy absorber means and/or movement means such as means known as a “silent block”.
Consequently, the main gearbox and/or the engine may be subjected in operation to relative movements, e.g. oscillations. The movements may be caused by the operation of the main gearbox and/or of the engine, or indeed by deformation of the structures supporting them.
As a result the members of the drive train may be moved out of alignment with one another, and that can lead to them being damaged.
In order to limit such misalignments, the power plant may include a shaft tube (named “trompette” in French language) surrounding the drive train. The tube is then rigidly fastened to the main gearbox and to the engine. The function of the tube is to tend to keep the spacing between the main gearbox and the engine constant, and to ensure sufficient alignment of the various members of the drive train. For example, a shaft tube may serve to align a transmission shaft with an inlet shaft of the main gearbox and with an outlet shaft of the engine.
Nevertheless, the risk of misalignment remains.
In order to accommodate possible misalignment between the transmission shaft, the outlet shaft of the engine, and the inlet shaft of the main gearbox, the drive train may include at least one resilient coupling means. Flexible coupling means may be of the type known as a “flector”, for example. Other coupling means may be envisaged, for example means of the type described in Document U.S. Pat. No. 7,959,512.
Each flexible coupling means serves to transmit the driving torque generated by the engine to the main gearbox. Furthermore, each flexible coupling means deforms in the event of axial or angular misalignment so as to make such misalignment possible and so as to limit the impact of the misalignment on the transmission shaft. Under such circumstances, flexible coupling means may be subjected to high force levels and may deteriorate progressively. Such coupling means are therefore monitored regularly.
Furthermore, the transmission shaft may also be designed so as to present sufficient flexibility in order to accommodate a predetermined amount of misalignment.
Under such circumstances, a power plant may include a shaft tube for limiting the presence of misalignments, and may include flexible coupling means so as to ensure that any misalignment that does occur has limited consequences on the power plant.
Such a system is advantageous but difficult to design. It may require testing in order to evaluate the misalignments to which the drive train is going to be subjected. In particular, inertial movements of an engine in flight can be difficult to assess without complex testing.
For ecological and economic reasons, for example, it may also be envisaged to make use not of a turbine engine but rather of a piston engine, e.g. a diesel engine.
Nevertheless, because of the presence of reciprocating members, a piston engine is subjected to different movements while starting, while accelerating, while operating even at a stabilized speed, or while being loaded with torque. These movements are then liable to degrade the physical integrity of the transmission shaft. These movements appear to be difficult to assess in the absence of testing, in comparison with the more linear movements of a turbine engine.
Document JP 2000-065265 does not form part of the technical field of the invention since it relates to a device for limiting shear and twisting forces in a bellows.
Documents FR 2 232 481, FR 1 388 363, U.S. Pat. Nos. 3,383,933, 3,034,583, and 2,718,756 are also remote from the invention.